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Comment by OneTimePetes | Hacker News Reader

This is.. true for artifacts entering the earth relative to the solar systems, aka meteorites that are already circling the sun at moderate speed - aka the earth on its orbit moves into them.

This does not hold for interstellar objects, which may come crashing into our spiralling course from any angle. Even for those, the likelihood for a "frontal" collision is slightly increased though.

Question: If a object comes at just the right angle.. could it essentially make a hover-touch down landing (on a planet with not much atmosphere)? As it would have not much motion relative to the planet system (okay, rotation should be very little) and gravity would add speed..

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  > If a object comes at just the right angle.. could it essentially
  > make a hover-touch down landing (on a planet with not much atmosphere)?

No. If the object were to come into just-barely contact with the surface of an airless world, then that body would have enough velocity at that point to take it back out to its apogee. And so does the planet, for that matter. For a hover-touch down landing, the relative speeds between the objects would have to be quasi-zero, so that friction could take over to set it down. But if the relative speeds between the objects were to be quasi-zero, then that would mean that the two objects are already in very, very similar orbits. As in, the apogee of the object would coincide in space (and time) with the apogee of the planet.

Plus, when such an object that is moving so (relatively) slowly approaches the planet, it will fall into the gravity well and pick up a lot of speed - the escape velocity of the planet, which for Earth is about 11.2km/s. So effectively there is a minimum speed at which an object can touch the planet's surface, if you disregard air resistance and assume that the object came from a long way away without doing anything funky like slingshots on its way in.

I can't see gravity perfectly canceling out forward velocity while simultaneously contacting the surface. It would have to come through the body first. Which, I suppose, is terrifyingly possible.

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